Another thing I wonder. How new is your "newest addition"? And has the pooping become worse since this extra cat came to live with you? Just another possibility.

Oh, and don't rule out a medical problem as a possibility. I have been seeing poops outside the box lately. I dismissed it. THEN I saw Lucas pee on the rug. I have since been treating him for a UTI and I have not been finding poops on the carpet any longer. since I never caught the pooper, I am only assuming that these incidents might have been related.